In the previously conducted Phase III clinical trials, Cycloset® (up to a maximum dose of 4.8 mg/day), administered either as monotherapy or combined with sulfonylurea therapy, significantly reduced HbA1c, fasting and post-prandial glucose and fasting and post-prandial triglycerides in obese individuals with type 2 diabetes mellitus. Clinical studies that combined Cycloset® with metformin were not as part of the original Cycloset® clinical program because metformin was not commercially available in the United States at the time that the studies were initiated. The present study is designed to investigate the efficacy and safety of Cycloset® compared to placebo when added to metformin monotherapy in persons with type 2 diabetes mellitus who are not adequately controlled on metformin therapy alone.